The present invention relates to an anaerobic treatment process for digesting organic substrates and producing residual solids containing reduced concentrations of pathogens. More specifically, this invention pertains to a method of pasteurizing sludge to bring the sludge to within exceptional quality as defined by the Environmental Protection Agency while maximizing solids destruction through the pasteurization.
Anaerobic digestion is used to convert a variety of organic materials to gas and soluble constituents. Anaerobic digestion is commonly used to convert a fraction of sewage sludge, animal manure, and other putrid organic slurries containing substantial quantities of pathogenic bacteria, viruses, fungi, and parasites to methane gas and soluble products. Conversion of sewage sludge, or other waste solids to gas and soluble constituents lowers the cost of solids disposal by reducing the mass of solids requiring disposal. Examples of processes that are useful for converting organic materials to gas and soluble constituents include the retained biomass processes described in U.S. Pat. Nos. 5,015,384 and 5,670,047 by the inventor of the subject application. The ""384 patent describes a retained biomass process similar to the one illustrated in FIG. 1. In such a process, an influent slurry 1 is delivered at a rate Q to anaerobic reactor 8. Solids, comprising partially digested solids and anaerobic bacteria, and liquid and soluble products of digestion are delivered via line 2 to a separator 9 where the solids are separated from the liquid and soluble products of digestion. The ""384 patent describes that the solids are either returned to anaerobic reactor 8 or they can be wasted via line 5 for disposal. Effluent from the separator can be discharged via line 4.
Retained biomass systems substantially improve the economics of anaerobic digestion. The digestor size can be significantly reduced while improving the percent solids converted to gas and soluble products. In a retained biomass system, solids must be wasted from the system on a periodic basis to avoid their undesirable build up. Referring to FIG. 1 and 2, solids can be wasted from separator 9 along line 5 or from the thickened return solids stream 3 shown in FIG. 1. Solids can also be wasted from the digestor 8 or from line 2 as shown in FIG. 2. Typically, it is more economical to waste the concentrated solids from the thickened solids stream 3 or directly from the separator 9 shown in FIG. 1, as compared to wasting the less concentrated digestor solids as shown in FIG. 2.
In addition to the volume and mass of residual waste solids, the cost of disposing waste solids is also influenced by the quality of solids requiring disposal. Solids, which have lower concentrations of bacteria and viral pathogens, can be disposed at less cost than solids containing higher concentrations of pathogenic organisms. Current regulations (EPA-40 CFR Part 503) define Class A or xe2x80x9cexceptional qualityxe2x80x9d biosolids as those waste solids containing a reduced quantity of pathogenic bacteria such as salmonella, and fecal coliform. Class A biosolids can be produced through a variety of means including:
1. Disinfection by holding the waste solids (biosolids) at an elevated temperature (above 50xc2x0 C.) for a prolonged period of time. Shorter detention times are achieved at higher temperatures. Pasteurization is achieved by retaining the solids or slurry for 30 minutes or more at 70xc2x0 C. or higher.
2. Raising the pH above 12 for 72 hours at an elevated temperature, or drying to 50% solids concentration.
3. In vessel or aerated composting for three days at 55xc2x0 C.
4. Windrow composting at 55xc2x0 C. for 15 days
5. Heat drying at 80xc2x0 C.
6. Heat treatment of liquid biosolids to 180xc2x0 C. for 30 minutes.
7. Aerobic thermophilic digestion at 55xc2x0 C. for 10 days
8. Beta ray irradiation
9. Gamma ray irradiation
As can be observed from the list above, the waste biosolids must have the pH or the temperature elevated for prolonged periods of time to achieve the desired pathogen reduction. The cost of achieving the desired pathogen reductions according to the above methods is directly related to:
1. The energy or chemical cost required to elevate and maintain the temperature of the waste biosolids; and
2. The reactor size required to hold the waste solids for the required period of time.
The quantity of chemicals used, the energy required, and holding vessel size, are each dependent on the quantity of solids that must be processed to reduce the pathogen concentration.
Specific methods of producing anaerobically digested xe2x80x9cClass Axe2x80x9d biosolids include the following processes:
1. Plug flow anaerobic thermophilic digestion or simulated plug flow digestion through a series of completely mixed thermophilic digesters. In this type of process the entire influent flow is heated to thermophilic temperatures. Heat losses from the digestors can be significant, since the volume to surface area required for multiple digestors is low. High effluent organic acids and ammonia concentrations, and the poor dewaterability of the residual solids are significant disadvantages of anaerobic thermophilic digestion or simulated plug flow digestion.
2. Batch pasteurization of the entire influent flow prior to anaerobic digestion is a commonly used practice. However, pasteurization of the entire influent flow prior to anaerobic digestion is expensive since the entire viscous flow must be pasteurized. The procedure requires large quantities of energy for transporting and heating the viscous influent slurry.
3. Thermophilic anaerobic digestion followed by mesophilic anaerobic digestion (Dague, et al. #5,746,919) has been described as useful to achieve Class A biosolids. However, in order to meet the strict requirements of EPA-40 CFR Part 503, in such a process the thermophilic digestor should operate in a plug flow mode to prevent short-circuiting, i.e., passage of a portion of the biosolids without being exposed to the treatment conditions for a sufficient period of time to effect the desired results. Again, with this type of process, the entire flow of biosolid stream must be heated to thermophilic temperatures and maintained at those temperatures for a considerable period of time, generally exceeding 7 days in the thermophilic reactor and 12 days in the mesophilic digestor. Such long retention times and large volumes contribute significantly to the overall cost of the process. In addition, two biomasses, mesophilic and thermophilic, are grown, thus reducing the mass converted to gas while increasing the mass to be disposed.
4. Thermophilic aerobic digestion followed by mesophilic anaerobic digestion also provides a process to achieve Class A biosolids. However, in order to meet the strict requirements of EPA-40 CFR Part 503, in this process the thermophilic aerobic digestor is generally operated in the plug flow mode or as a sequencing batch reactor to prevent short-circuiting. Again the entire flow must be heated to thermophilic temperatures and maintained at those temperatures for a considerable period of time, generally exceeding 5 days in the aerobic reactor, and 12 days in the anaerobic digestor. Costs associated with the heating, transporting and storing the process stream can be significant. In addition, a significant portion of the methane gas generating potential is lost in the aerobic portion of such process.
5. U.S. Pat. No. 5,888,453 describes a continuous flow pasteurization process for sewage sludge. The process described in the ""453 patent involves raising the temperature of the sludge by passing the sludge through at least one heat exchanger supplied with a heating fluid. The sludge at the elevated temperature is delivered to a detention tank where it is held above a predetermined temperature for a predetermined period of time to effect pasteurization. The ""453 patent describes that the pasteurized sludge can be fed to a second anaerobic reactor for further digestion, followed by delivery to a dewatering system.
There continues to be a need for an energy efficient anaerobic digestion process capable of effectively and efficiently digesting organic substrates and producing residual solids that contain reduced quantities of pathogens so as to be classified as Class A or xe2x80x9cexceptional biosolidsxe2x80x9d by the EPA.
The present invention provides a method for anaerobically digesting organic substrates and producing residual solids that contain pathogens at a level that allows the residual solids to be classified as Class A biosolids. The method concentrates digested products which reduces the volume of material that must be treated to reduce the pathogen levels. Reducing the volume of material that must be treated provides cost savings associated with reduced energy costs and equipment costs. Direct contact with a hot or inhibitory fluid is relied upon to treat the residual solids to reduce the pathogen levels. Preferably, steam is used as a relatively inexpensive source of heat and also provides a source of fluid to dilute the residual solids so that they can be subsequently treated in an additional anaerobic reactor to further reduce the solids volume. Gases such as air, oxygen, nitrogen, hydrogen, methane, and carbon dioxide may also be used as a source of heat either alone or in combination with other gases. Inhibitory gases such as ammonia, hydrogen sulfide, ozone and in some cases oxygen may be used to cause the pathogens to undergo lysis or used as a heat source alone or in combination with other gases.
An anaerobic process in accordance with the present invention includes the step of digesting an organic substrate in a first anaerobic reactor to provide reactor contents comprising solids and liquid products of digestion. The process includes a step of removing a portion of the reactor contents, and a step of pasteurizing all or part of the removed reactor contents by direct contact with a hot or inhibitory fluid.
In a preferred embodiment of an anaerobic process in accordance with the present invention, the process includes the steps of; digesting an organic substrate in a first anaerobic reactor to provide reactor contents comprising solids and liquid products of digestion, removing a portion of the reactor contents, concentrating the portion of the removed reactor contents by separating liquids, returning concentrated reactor contents to the first anaerobic reactor, pasteurizing any leftover concentrated reactor contents by direct contact with a hot or inhibitory fluid, and digesting the leftover reactor contents that underwent pasteurization in a second anaerobic reactor. In the preferred embodiment, the fluid to bring the leftover concentrated reactor contents to the desired temperature may be either in a gaseous or liquid phase or any combination thereof. If the fluid is in a gaseous phase, the gaseous phase may be either steam, air, nitrogen, oxygen, carbon dioxide, hydrogen, hydrogen sulfide, ozone, methane or ammonia or any combination thereof. If the fluid is in a liquid phase, the liquid phase may be water. The fluid may also have disinfectant chemicals.
In another embodiment of the present invention, the anaerobic process includes the steps of; digesting an organic substrate in a first anaerobic reactor to provide reactor contents having solid and liquid products of digestion, removing a portion of the reactor contents, concentrating removed reactor contents by separating liquids, returning concentrated reactor contents to the first anaerobic reactor and pasteurizing any leftover concentrated reactor contents by direct contact with a hot or inhibitory fluid. During pasteurization, gases such as ammonia or hydrogen sulfide may be given off and reintroduced into the process. Since the pasteurized reactor contents and the gases are substantially in equilibrium, reintroducing part or all of the gases contributes to the heat energy required for pasteurization, thus, conserving energy. In addition, the nature of these gases causes the cells to undergo lysis. In this embodiment, the fluid may have a gaseous or a liquid phase or any combination thereof. If the fluid is in a gaseous phase, the gaseous phase may be either steam, air, nitrogen, oxygen, carbon dioxide, hydrogen, hydrogen sulfide, ozone, methane or ammonia or any combination thereof. If the fluid is in a liquid phase, the liquid phase may be water. The fluid may also have disinfectant chemicals.
Using the processes in accordance with the present invention, exceptional quality, xe2x80x9cClass Axe2x80x9d biosolids may be achieved efficiently and economically.